Historically, access to computing resources and data may be limited to authorized users. Typically, an authorized user may be authenticated by a username-password pair. The password is kept confidential, but storage of passwords on a host presents a risk of compromise, for example, by a hacker. Two-factor authentication improves security, but still requires a password and adds additional burden on the user. Passwords are increasingly problematic because complex passwords are hard to remember for humans; people reuse passwords at several sites, thus increasing the risks; and forced rotation of passwords for compliance reasons often ends up weakening security of the system. For internal users such as employees and contractors, turnover, new hires, and the like complicate the challenge of controlling access across large numbers of servers. The need remains for improvements in user authentication.